Microelectronic packages include components for forming electrical connections between microelectronic devices such as microelectronic chips or wafers and external circuitry. microelectronic packages commonly incorporate conductive elements such as fine metallic leads and terminal structures disposed on a dielectric layer such as a polymeric layer. In conventional tape automated bonding or "TAB" a prefabricated array of leads is provided on a flexible dielectric tape. The leads are bonded to contacts on the microelectronic device. Certain embodiments illustrated in U.S. Pat. Nos. 5,489,749; 5,518,964; 5,148,265; 5,148,266, and 5,491,302 and International Publication WO 94/03036, the disclosures of which are hereby incorporated by reference herein, also incorporate leads and other electrically conductive elements. Leads for use with modern semiconductor chips having large numbers of closely-spaced contacts must be very fine. They may be about 20 to about 40 microns wide. These leads must be provided in precise locations on the connection component. Other components such as circuit panels also include fine metallic features such as conductors.
The metallic elements in these and other components have been fabricated by various processes, most commonly by photochemical processes. In one photochemical process, patternwise etching of a metallic layer is utilized to form the leads. A photographically patterned etch resist is used to selectively etch unwanted portions of the metal layer so that the resist-protected portions form the leads of the connection component. In another method, a metal is plated in areas defined by a photographically patterned resist.
Photochemical processes suffer significant drawbacks in that they require several steps. The resist must be exposed to illumination in the desired pattern, typically by use of a mask. The resist is thereby developed so as to cure only the resist in exposed areas or only the resist in the unexposed areas. The uncured resist is then removed, leaving a mask which has openings in areas where metal is to be removed or added. After etching or plating, the cured resist forming the mask is then removed. These steps entail significant cost and limit the speed of fabrication. Electroplating produces unacceptable irregularities in the metal elements formed when performed too rapidly, also limiting the speed with which the leads may be fabricated. In addition, photochemical processes typically cannot form features smaller than a certain size. This size depends on the type of resist used and the developing process.
Conventional stamping processes have been used to fabricate relatively large metallic elements such as large leads. In a simple stamping process, a sheet of metal is passed between a pair of matched tools referred to as a punch and a die. The punch has a protrusion corresponding to the shape of the part to be formed, whereas the die has a hole precisely matched to the shape of the punch, and just slightly larger than the punch. As the tools are forced together, the punch enters the hole in the die and shears a portion of the metal sheet corresponding in shape to the punch from the remainder of the sheet. In variants of these processes, the tools may perform additional operations such as bending the parts. Stamping processes can be performed rapidly. Although stamping processes can be used to form relatively large, coarse parts, it is typically not practical to stamp very fine leads for use with microelectronic connection components having closely spaced contacts.
Thus, despite the substantial time and effort expended to solve the problems associated with fabrication of leads used in packaging microelectronic elements and other metallic parts, further improvement in such processes would be desirable.